This invention relates to a system and method for numerically controlled cutting of pieces from sheet material, and more specifically for accurately cutting pieces from a closely packed marker.
Numerically controlled cutting machines are widely used in various industries for cutting various limp sheet materials such as woven and non-woven fabrics, vinyl and other plastics, paper, cardboard, leather, etc., as well as solid materials like sheet metal, lumber, glass, etc. The cutting tool cuts either a single sheet of material or a stack of multiple sheets (multi-ply layups) under the control of a microprocessor, which is called a numerical controller. An example of such a system for cutting limp sheet material, as disclosed in the U.S. Pat. No. 4,327,615 to Heinz Gerber et al is discussed in the preferred embodiment section of the current invention (see FIG. 1). The numerical controller converts data, written in a specific format, into signals that moves the cutting tool with the given speed along the given tool path, defined by the X, Y and Z coordinates of some reference point of the cutting tool. The numeric control (NC) data define the so-called nesting or layout of pattern pieces, that is the shape and location of the pattern pieces in a marker, the marker being a set of pattern pieces, or templates.
In order to save material, pieces in the marker are routinely positioned closely to each other; frequently touching or even slightly overlapping each other, as shown in FIG. 2. Referring now to FIG. 2, a number of templates 7 are nested together to form a marker 8, which represents the pieces to be cut out of the sheet material.
It is well known in the art that closely nested pieces are much more difficult to cut compared with loosely packed pieces. Situations that create problems when cutting are called xe2x80x9ccriticalxe2x80x9d situations; regions within the marker that give rise to critical situations are called xe2x80x9ccriticalxe2x80x9d, or xe2x80x9csensiblexe2x80x9d regions; and portions of the tool path (straight line segments and/or points) that are difficult to cut properly are called xe2x80x9ccriticalxe2x80x9d or xe2x80x9csensitivexe2x80x9d lines, or portions, or segments, or points.
It is well known in the art that cutting problems are most profound near the points of tangency or close approach (FIGS. 3A-3C) and near common lines (FIGS. 4A-4D). The major difference between these two xe2x80x9ccriticalxe2x80x9d situations is the magnitude of the angle between the xe2x80x9ccriticalxe2x80x9d lines and the length of the portion of a xe2x80x9ccriticalxe2x80x9d line that is so close to another xe2x80x9ccriticalxe2x80x9d line that cutting this portion of the first line after the previous line has been cut presents a problem (FIG. 5).
To be classified as common lines, the angle between xe2x80x9ccritical linesxe2x80x9d should generally be small, no more than several degrees, while according the U.S. Pat. No. 4,327,615 the angle between tangent lines can be as great as 30 degrees. The xe2x80x9ccriticalxe2x80x9d portion of each common line must be, as a rule, much longer, typically several inches or more, while for two tangent lines lengths in the order of tenths of an inch might be enough. It should be mentioned that the common lines geometry could vary from xe2x80x9cexternalxe2x80x9d common lines between neighboring pieces, as shown in FIG. 4A to xe2x80x9cinternalxe2x80x9d common lines between overlapping pieces, as shown in FIG. 4B. Referring to FIG. 4A, two templates, 41 and 43, have two sides, 42 and 44, which are in proximity to each other, but do not actually touch. If these sides are within a few tenths of an inch from each other, they may be treated in the same way as if they were common sides. Referring to FIG. 4b, two templates, 51 and 53, contain sides 52 and 54, which overlap. Side 52 may be considered internal to template 51, but if the overlap is within the order of a tenth of an inch, this situation may be treated as if the two sides were common. There may exist similar varieties in between these conditions, as shown in FIG. 4C, in which template 61 has side 62 which is actually common with side 63 of template 64 for most of its length. Referring now to FIG. 4D, template 71 contains side 72 which is common with side 74 of template 73, except that in this case the length of commonality is only about one half the length of the longer side 74.
The tangency geometry could vary as well: it can be an xe2x80x9cunidirectionalxe2x80x9d (xe2x80x9cone-sidedxe2x80x9d) xe2x80x9ctangentxe2x80x9d point (FIG. 3A), or a xe2x80x9cbi-directionalxe2x80x9d (xe2x80x9ctwo-sidedxe2x80x9d) xe2x80x9ctangentxe2x80x9d point (FIG. 3B), both considered in U.S. Pat. No. 3,864,997 to Pearl and Robison, or a point of close approach (not a classical tangent point at all, but in spite of that usually called a xe2x80x9ctangentxe2x80x9d point anyway), discussed in U.S. Pat. No. 4,327,615 to Gerber (FIG. 3C). In further discussion we usually use the terms xe2x80x9ccommon linexe2x80x9d and xe2x80x9ctangencyxe2x80x9d to describe all those varieties, though sometimes, when confusion is possible, we call them xe2x80x9cgeneric common linexe2x80x9d and xe2x80x9cgeneric tangent pointxe2x80x9d (xe2x80x9cgenericxe2x80x9d meaning any variety).
Cutting xe2x80x9ccriticalxe2x80x9d lines may result in reduced cut quality and/or even in damaging the cutter. For example, when cutting a limp sheet material a cutting blade severs the limp material as it advances along the cutting path but does not remove the material. As a result, the material is pushed aside by the advancing blade and generally flows around the cutting blade in pressing engagement. This pressure, combined with the ability of the layers of limp material to move against each other, forces the blade to deviate from the programmed line of cut toward the direction of xe2x80x9cless resistancexe2x80x9d. According to Heinz Gerber (U.S. Pat. No. 4,327,615), xe2x80x9cwhen a cutting blade passes in close proximity to an adjacent pattern piece that was cut at an earlier stage in the operation, the kerf created by the previous cut interrupts the continuity of the limp sheet material and allows the material at one side of the knife blade to yield more easily to the blade than at the opposite side. As a result, the blade experiences unbalanced lateral loadingxe2x80x9d. Apparently, the closer the cutting path approaches the previous cut, the greater the unbalanced loading and the blade bending will be. The blade may eventually break up or jump completely into the kerf of the previous cut. Inaccuracies or damage to the machine are the ultimate consequences.
It is believed that the above-described condition arises for tangent points (including points of close approach) as well as for common lines. That is why it is difficult to cut all of them properly. Cutting one of common lines after the other common line has been cut can also result in frying of the material along the cut, thus resulting in a more severe cutting problem than in the tangency situation, especially when the two common lines are strictly coincident.
Similar problems, though for different reasons, arise when cutting solid materials. For example, cutting a sheet metal may produce extra internal tension, create extra defects, change the planar form of the sheet, and/or modify its elastic properties, etc., depending on the given type of the metal and the chosen cutting tool. All these changes may (and usually do) propagate within some region around the cut. Therefore cutting the metal within this area second time may (and does) result in various cutting problems, specific for each material type/cutting tool combination.
Several approaches have been suggested to overcome the difficulties associated with tangencies and/or points of close approach (FIG. 3) between closely packed pieces.
In U.S. Pat. Nos. 3,855,887 and 3,864,997 Gerber reveals that in such a xe2x80x9ccriticalxe2x80x9d cutting area a reciprocal knife blade may be slowed down with reduced feed rate signals and/or rotated out of tangent position with yaw signals, the signals being introduced manually by the cutter operator. In U.S. Pat. No. 4,327,615 Gerber proposes to add slow down and/or yaw command(s) to the NC data with the so-called preprocessing means that is with the help of a computer before feeding the data into the cutter. In addition, the above-mentioned patent suggests adding translation commands to NC data that guide the cutting blade along a path offset slightly from (away) the path at a pattern piece periphery, thus increasing the buffer between pieces within the xe2x80x9ccriticalxe2x80x9d region by changing the xe2x80x9ccriticalxe2x80x9d portion of the tool path. This approach works well for xe2x80x9ccriticalxe2x80x9d regions created by points of tangency or close approach, although changing the direction of the cut, as explained in the U.S. Pat. No. 3,864,997, produces better results when it is applicable.
The current invention resolves a problem which none of the three approaches by Gerber (slowdown, yaw signal or buffer increasing translation) solves, in regard to the cutting of common lines. Slowing the blade down results in diminished throughput, and while slowing down the knife along a short path near the tangent point is acceptable, systematic slowing down along all common line paths is not desirable. Besides, slowing down the knife moderately along the long common line is usually just not enough to avoid complications caused by the accumulation of the unbalanced lateral loading effect during a long path. The application of the yaw signal for a long enough period of time is usually insufficient. Increasing buffers between pieces by decreasing the piece area (buffer increasing translation) may be acceptable for point-like critical situation, where the spatial dimensions of a critical region are small compared with the piece dimensions. However, substantial reduction of the piece area by changing the piece border along the common line when the typical dimensions of the critical region are the same as the dimensions of the piece itself is usually unacceptable (otherwise the piece would have the smaller area from the very beginning).
The inventions revealed in U.S. Pat. No. 3,495,492, U.S. Pat. No. 3,855,887, U.S. Pat. No. 3,864,997, and U.S. Pat. No. 4,327,615, all to Gerber et al., deal with the cutting of pieces that are positioned outside of each other. The boundaries of those pieces can closely approach each other in a critical region of a relatively small size, or even touch each other in a tangent point, but they never overlap each other, the overlapping problem being outside the scope of those inventions. The tool path problems in all those cases are essentially solved by either changing the operation mode of the blade (slowdown, blade spatial orientation, cut direction, etc.), or by changing direction of the cut, or by increasing the buffers by reshaping pieces.
There is another vast area of prior art that is concerned with the cutting of overlapped pieces but does not deal with other tool path problems like cutting a line in a close proximity of a previously cut line. As mentioned by Loriot in U.S. Pat. No. 4,819,529, xe2x80x9cin some particular applications it may be acceptable, or indeed desirable, to allow pieces to overlap during placing so long as the overlaps do not significantly spoil the quality of the finished product. For example, this may save raw material. Also, pieces overlaps may be the result of inaccurate placing or of an error in the system for inputting the positions of the pieces when such a system is used in the cutting process.xe2x80x9d
The U.S. Pat. No. 5,703,781 presents a case where overlapping results from inaccurate placement of the pieces during the first phase of the nesting process and is corrected in the second phase of the said nesting procedure. In the U.S. Pat. No. 5,703,781 Charles Martell et al. reveal an automatic marker making system and method in which the creation of a new marker is facilitated through the use of already existing marker designs. A computer database of existing markers is searched for markers that are xe2x80x9csimilarxe2x80x9d to the marker being created. Initially, position and orientation data from pattern pieces in the xe2x80x9csimilarxe2x80x9d marker are used to position and orient corresponding pieces in the new marker. The new marker is then xe2x80x9ccompactedxe2x80x9d using a software routine to nest all of the new pieces. The compacting routine corrects the overlaps between pieces by moving pieces in the marker without changing the shapes (boundaries, etc.) of the pieces. New positions of pieces are determined by solving a non-linear combinatorial optimization problem with restrictions.
The U.S. Pat. No. 3,596,068 to Doyle reveals a system for optimizing material utilization, where he is using data processing means xe2x80x9cto simulate a non-interfering translation of the piece in tangential contact with the marker boundary.xe2x80x9d Similar to U.S. Pat. No. 5,703,781, he uses translations in order to avoid overlapping, thus reducing the overlapping problem to the problem of nesting.
It is evident that prior art discussed previously strives to remove overlap between pieces by moving pieces, thus reducing the overlap problem to the so-called nesting problem (described, for example, in the U.S. Pat. No. 5,703,781 to Martell et al. and references therein). At the current level of computer technology, any known computer-software solution to the nesting problem, in particular, a solution by Milenkovic et al., cited in the U.S. Pat. No. 5,703,781 appears to produce inferior results compared to the results manually obtained by experienced human operators. Moreover, even if translation successfully corrects overlaps between pieces, it rarely space between pieces and creates xe2x80x9ctangenciesxe2x80x9d, xe2x80x9ccommon linesxe2x80x9d and all other critical conditions that Gerber et al. were trying to solve in their patents. On the other hand, if translation does create buffers, it wastes the material, which is extremely undesirable, since the cost of the material is the major part of the overall cost of the production.
All prior art discussed so far is devoted to cutting multi-ply layups of sheet material with an automatic and numerically controlled cutter. The problem of nesting of overlapping pieces is important in many other cutting processes utilizing various cutting machines, including manual cutting of one sheet of a material with a knife by a human worker. It is especially true for cutting hides and leather with natural defects, where overlaps may be acceptable, or indeed desirable, in order to save precious raw material.
For example, in U.S. Pat. No. 4,819,529, Loriot reveals a method, and in particular an automatic method, of cutting parts out from sheet or plate material. The method comprises cutting out parts from sheet or plate material along outlines defined by piece templates; it includes an improvement in which any overlaps between templates are detected and the lines of cut where the templates overlap are modified either by cutting along a straight line interconnecting the points of intersection between the outlines of the overlapping templates, or by cutting along an average line equidistant from the outlines of the templates between the points of intersection of the outlines of the overlapping templates, or else by cutting along the outline of one or other of the overlapping templates, with the type of cut being selected for each overlap zone as a function of the types of the overlapping templates and of the portions of template outlines concerned, the said selections being suitable for storage in a list of possible types of cut, which list may be consulted immediately after detecting and identifying a given overlap. These overlap operations may be performed by a computer. U.S. Pat. No. 4,819,529, similar to the previously discussed U.S. Pat. No. 5,703,781 to Charles Martell et al., does not deal with situations like tangencies or (at least, external or strictly coincident) common lines, probably because those cases are not crucial for manual cutting of one layer of a material. Besides, Loriot""s solution results in an undesirable cutting path as soon as overlapping geometry becomes even moderately complex, for example when a line intersects a saw-like boundary. Moreover, Loriot does not even consider the cut sequence in which a new equidistant line must be cut with respect to other lines of the intersecting pieces, thus avoiding the dry haul (moving the blade in the air without actual cutting) and similar optimization problems at all, probably, once more, because the cutting protocol is not important for manual cutting.
In U.S. Pat. No. 3,864,997, Pearl and Robison reveal a system and method for cutting multiple pattern pieces from a layup of sheet material in which contour segments of individual pieces are cut in different directions (clockwise and counterclockwise). The point on any given pattern piece toward which a cutting blade is advanced from different directions is generally the point of closest approach to an adjacent or contiguous pattern piece in the marker. The program generated by the above-identified system also permits certain contour segments to be cut before others. As a result, it allows the tool to approach xe2x80x9csensitivexe2x80x9d points, such as a point of tangency or a point closest to the contour of an adjacent piece, from two directions and to alleviate difficulties by making certain cuts before others. The feed rate and tangency of the cutting blade are also regulated at sensitive cutting points such as the points of closest approach to an adjacent pattern piece. When revealing the preferred embodiment of their invention, Pearl and Robison also consider a special cutting situation of strictly coincident common lines, which is illustrated in FIG. 4D where pattern pieces D and E are contiguous between points 78 and 79. In order to save time during the cutting operation and to avoid fraying of the fabric material along the previously cut segment, they discuss two possible solutions: either the xe2x80x9cfirst-takes-allxe2x80x9d approach, when the common line segment is omitted entirely from the piece that is cut second; or the xe2x80x9cnobody-winsxe2x80x9d approach, when the combined profile of pattern pieces D and E is cut in its entirety and then the common contours of the pattern pieces are cut with a single pass. Unfortunately for the industry, these simple and well-known approaches (see, for example, a similar technique mentioned in the U.S. Pat. No. 4,819,529 to Loriot in connection with overlapping) cannot be easily extended to more complex and realistic situations, for example, when common line segments do not strictly coincide, or when more than two pieces have common lines. It must also be mentioned that in the above-discussed case of a xe2x80x9cstrict common line between two piecesxe2x80x9d, as a rule, the common line must be cut first, in contrary to the version of xe2x80x9cnobody winsxe2x80x9d cutting protocol suggested by Pearl and Robison.
Nevertheless, despite all of the above improvements in the prior art, there still remain a number of situations in which the commonly used technique requires manual intervention in the numerical control program. These problem situations typically involve adjacent templates within the marker in which there are points of tangency, and in which there are common lines between adjacent markers.
It is necessary to first detect such circumstances and then to xe2x80x9cfixxe2x80x9d the detected tool path problems. This detection is generally done in the prior art by a visual inspection of the marker by skilled operators. The operator will then identify portions of the NC program where these problem situations occur, and try to solve the detected tool path problem by manually (interactively) changing the knife path, or manually (interactively) changing the speed of the knife.
It is, accordingly, a major objective of the present invention to provide a system and method to automatically identify and classify critical cutting conditions called generic tangencies (including points of closest approach) and/or generic common lines (internal and/or external, strictly or approximately coincident), and to then automatically guide a cutting blade past such critical cutting conditions without damaging the cutter or substantially sacrificing quality or throughput by automatic preprocessing of data defining a marker.
In accordance with the present invention, whenever a marker consists of pieces that have one or more generic tangencies or generic common lines, the marker is pre-processed as follows: (1) tangencies and common lines are detected and classified; (2) tangencies are resolved using well known algorithms of prior art; (3) common line segments are eliminated using algorithms of the current invention: pieces with common line segments are reshaped so that the largest possible portions of the tool path become strictly coincident while buffer between pieces is eliminated; after that coincident portions of the tool path created at the previous step are replaced by a newly created portion of the tool path, so that each common line path is cut once instead of twice; (4) the new tool path is generated so that the best possible quality and highest possible throughput are achieved. Note that the highest quality requirement usually means that the newly created common line portions of the tool path are cut continuously, as a whole, without lifting and then reinserting the cutting tool, and before all other portions of the tool path.
The ability to automatically resolve generic tangency and common line critical situations results in following advantages:
(a) higher operator productivity because manual solution of these critical problems is very time consuming;
(b) better accuracy of the cut, by removing tool path deviation along the path of xe2x80x9cless resistancexe2x80x9d;
(c ) better quality of the produced pieces because of better accuracy and absence of frying and other damage to the material; and
(d) reducing the material waste, since pieces in a marker are intentionally packed more closely than in the prior art practice, with intentionally created critical problems to be resolved by post-processing of the NC data.
It is a general object of the current invention to provide an automatic method of cutting sheet material from a closely-packed marker containing tangency points and common lines. It is a specific object of the invention to provide such a method that minimizes cutting time.
In accordance with one aspect of the present invention, a method of cutting parts out from sheet by means of a numerically-controlled cutting system having a cutting tool which cuts along a path, includes placing a plurality of templates, each having a plurality of segments, having the shapes and sizes of the parts upon the sheet into a closely-packed marker, minimizing the spaces between the templates, then inputting the marker into a pre-processor. Within the processor are the steps of detecting tangencies and common lines between templates, and then changing the tool path and speed to solve the detected tangency and common line problems.
In accordance with a second aspect of the invention, the common line detection further includes the steps of detecting all proximate pair of segments, and then, for each proximate pair of segments, checking if said pair has an angle between segments smaller than a threshold angle, xcex2cr, and if so, then clipping each segment of the pair by the belt rectangle of the other segment and calculating the clipped length. Finally, if the clipped length is greater than a maximum allowable xe2x80x9cthresholdxe2x80x9d distance, Dcr, then the segments are marked as common line segments.
According to a third aspect of the invention, the segments are marked as tangent segments: (1) if the angle between segments is less than the maximum allowable angle, xcex1cr, (which may and usually is different from the maximum allowable angle, xcex2cr, used in the common line detection algorithm); (2) if the segments are not common line segments; and (3) if the clipped length is greater than the maximum allowable xe2x80x9cthresholdxe2x80x9d distance Lcr (which may and usually is different from the maximum allowable xe2x80x9cthresholdxe2x80x9d distance, Dcr, used for detection of common lines).
According to a fourth aspect of the invention, the path and speed of said cutting tool are determined by a numerical control program.
According to fifth aspect of the invention, the changing of the tool path is done by a cutting operator, by printing the marker out to a drawing or by viewing and measuring the marker on the screen, then cutting pieces manually.
According to a sixth aspect of the invention, each common line is cut in one pass.
According to a seventh aspect of the invention, each common line may be cut manually in one pass.
According to a eighth aspect of the invention, each common line may be cut as one tool path segment, that is the cutting tool cuts the common line continuously without any dry haul and without lifting and reinserting the cutting tool.
According to a ninth aspect of the invention, at least one of common lines can be approximated by a straight line.
According to a tenth aspect of the invention, at least one of common lines can be approximated by a curved line.
According to an eleventh aspect of the invention, each curved common line is approximated by a sequence of a straight line segments.
According to a twelfth aspect of the invention, the creation of the closely-packed marker is done by a marker generation software.
According to a thirteenth aspect of the invention, the creation of the closely-packed marker is done by video scanning of a physical model of templates arranged within the area of a sheet of material.
According to a fourteenth aspect of the invention, all the templates are sorted into one or more subsets such that templates in each subset contain common segments with the templates of that subset only, and then each subset is sorted into sub-subsets of common lines segments such that each common line segment belongs to one sub-subset only. Then, for each sub-subset, a common line is created that approximates all the common line segments therein. Finally, the optimal tool path is calculated for each template containing a common line.
According to a fifteenth aspect of the invention, an optimum tool path is selected that minimizes intra-piece dry haul time.
According to a sixteenth aspect of the invention, an optimum tool path is selected which maximizes intra-piece quality by imposing additional constraints, like cutting common lines before the perimeter of the piece.
According to a final aspect of the invention, each common line is generated by a number of different methods, including straight line approximation, polynomial interpolation, least squares fitting, B-Spline interpolation, cubic spline interpolation, and a user-selected non-linear curve.